For effective provision of multimedia and broadcasting services, which are continuously increasing in volume, the development of passive optical network is being actively carried out. In particular, a WDM-PON technology believed to be developed in Korea exhibits excellent performance. Generally, equipments used therein were more expensive than those used in G/E PON (G-PON: gigabit passive optical network, E-PON: Ethernet passive optical network). Therefore, the technique has not been attracting attention in view of the less expensive competition. Recently, however, the cost has been considerably reduced with the development of gigabit technology, and the price per bandwidth may now be even cheaper than those of its competitors. Accordingly, the WDM-PON is attracting attention again.
In particular, as gigabit ONUs are provided, the transmission capacity of a WDM-PON has increased up to 2 gigabits in a case of 16 channels. A communication failure caused by disconnection of backbone network or distribution network optical fibers for connecting a CO to ONUs may cause a serious economic loss. Therefore, there is a need to develop a system which can survive even where a failure occurs in optical fibers.
FIG. 1 is a diagram showing the configuration of a conventional passive optical network which uses light sources having different wavelengths from each other. In the conventional passive optical network, a bypass optical fiber is added to recover communication where a failure occurs in an operational optical fiber. In the bidirectional WDM-PON system, a CO and an RN are connected through two operational and protective optical fibers 140 and 142 by using a 1×2 switch 130 and a 2×1 coupler 150. In this case, some of upstream signals between the switch and the operational optical fiber are extracted to monitor the upstream signals.
In a normal state, the CO sends a multiplexed optical signal through the operational optical fiber 140. Further, the RN receives the multiplexed optical signal from the operational optical fiber through the 2×1 coupler 150, and a 1×N AWG 160 demultiplexes the multiplexed optical signal to transmit to the respective ONUs through distribution optical fibers 161 and 163. Inversely, upstream optical signals output from the respective ONUs are transmitted to the RN, and are then multiplexed by the 1×N AWG 160 so as to be transmitted to the CO. Meanwhile, where a failure such as the disconnection of operational optical fiber occurs, an upstream signal optical fiber monitor 120 detects the failure and then switches the path of the 1×2 switch 130 into the protective optical fiber 142 to thereby recover communication.
In the conventional WDM-PON, a colorless ONU may not be utilized. Further, although the communication may be recovered where a failure occurs in the backbone network, it may not be recovered where a failure occurs in the distribution network. Recently, since ONUs operate at a gigabit rate and a large number of users are connected to one ONU, there is a need for a WDM-PON which may recover communication where a failure occurs in a distribution optical fiber.